1. Field of the Invention
The present invention relates to an air intake side secondary air supply system using a lean oxygen concentration sensor for an internal combustion engine equipped with a fuel increment control system.
2. Description of Background Information
Air-fuel ratio feedback control systems for an internal combustion engine are well known as systems in which oxygen concentration in the exhaust gas of the engine is detected by an oxygen concentration sensor (referred to as O.sub.2 sensor hereinafter) and the air-fuel ratio of mixture to be supplied to the engine is feedback controlled in response to an output signal level of the O.sub.2 sensor for such purposes of the purification of the exhaust gas and improvements of the fuel economy. As an example of the air-fuel ratio feedback control system, an air-intake side secondary air supply system for the feedback control is proposed, for example, in Japanese Patent Publication No. 55-3533 in which an open/close valve is disposed in an air-intake side secondary air supply passage which leads to the carburetor, on the downstream side of the throttle valve, and the open/close valve is opened or closed in response to an output signal level of the O.sub.2 sensor, i.e., a duty ratio control of the amount of the air intake side secondary air is performed. In conventional air intake side secondary air supply systems as this example, it was general to use an O.sub.2 sensor whose output signal does not vary in proportion to the oxygen concentration in the exhaust gas. On the other hand, a "lean" O.sub.2 sensor has been developed recently, whose output signal varies in proportion to the oxygen concentration in the exhaust gas if the air-fuel ratio of the mixture to be supplied to the engine is on the leaner side of a stoichiometric air-fuel ratio. Further, an air-fuel ratio control method of fuel metering type using the lean O.sub.2 sensor for precisely controlling the air-fuel ratio toward a target air-fuel ratio within a lean air-fuel ratio region is already known. For instance, this type of control method is described in Japanese patent application laid open No. 58 -59330.
On the other hand, in internal combustion engines, a fuel increment control system is generally provided for improving the driveability, which system is designed to increase an amount of fuel being supplied from the carburetor to the engine according to operating conditions of the engine, to enrich the air-fuel ratio of the mixture.
If an air intake side secondary air supply system using the above mentioned lean O.sub.2 sensor is provided in an internal combustion engine equipped with the fuel increment control system, it will become difficult to control the air-fuel ratio of the mixture precisely, to cause adverse effects to the driveability of the engine. More specifically, if the air-fuel ratio is controlled toward the target air-fuel ratio in the lean region while the fuel increment control system is in operation, the air-fuel ratio does not necessarily follow the target air-fuel ratio precisely. Further, engine output power may become insufficient in some occasions in various driving conditions of the engine. Thus, it is very likely that the driveability of the engine will be deteriorated by mere combination of the air intake side secondary air supply system using a lean O.sub.2 sensor and the fuel increment control system.